Recently, hybrid vehicles and electric vehicles have attracting attention as environmentally friendly vehicles. A hybrid vehicle is a vehicle having, as a power source, a DC power supply, an inverter and a motor driven by the inverter, in addition to a conventional engine. Specifically, the power source is obtained by driving the engine and, further, a DC voltage from the DC power supply is converted by the inverter to an AC voltage and the motor is rotated by the converted AC voltage, whereby the power source is obtained.
An electric vehicle is a vehicle having, as the power source, a DC power supply, an inverter and a motor driven by the inverter.
In the hybrid vehicle or the electric vehicle as such, a configuration has been studied in which the DC voltage from the DC power supply is boosted by a boosting converter, and the boosted DC voltage is supplied to the inverter for driving the motor.
FIG. 6 is a schematic block diagram showing an example of a conventional motor driving apparatus.
Referring to FIG. 6, the motor driving apparatus includes a main battery MB, system relays SR1, 2, a boosting converter 101, an inverter 102, a DC/DC converter 110, a subsidiary battery SB, and a control unit 120.
Main batter MB outputs a DC voltage. System relays SR1, SR2 supply, when turned on by a signal SE from control unit 120, the DC voltage from main battery MB to DC/DC converter 110.
Boosting converter 101 boosts the DC voltage supplied from main battery MB by the control from control unit 120, and supplies the boosted DC voltage to inverter 102.
Receiving the DC voltage supplied from boosting converter 101, inverter 102 converts the DC voltage to an AC voltage under the control by control unit 120, and drives motor generator MG. Consequently, motor generator MG is driven to generate torque designated by a torque command value TR. Current sensor 104 detects a motor current MCRT flowing in each phase of motor generator MG, and outputs the detected motor current MCRT to control unit 120.
DC/DC converter 110 lowers the DC voltage supplied from main battery MB through system relays SR1 and SR2, in response to a control signal from control unit 120, and supplies the lowered DC voltage to subsidiary battery SB. Subsidiary battery SB stores the supplied DC voltage and outputs a DC voltage for driving subsidiary electric components, not shown.
Based on the DC voltage of main battery MB, the motor current MCRT from current sensor 104 and the like, control unit 120 generates signals PWC, PWM for controlling boosting converter 101 and inverter 102, and outputs the generated signals PWC and PWM to boosting converter 101 and inverter 102, respectively. Further, control unit 120 generates a control signal for controlling DC/DC converter 110, and outputs the signal to DC/DC converter 110.
In this manner, the motor driving apparatus mounted on a hybrid vehicle or an electric vehicle boosts the DC voltage from main battery MB and drives the motor generator MG to generate prescribed torque, and lowers the DC voltage from main battery MB to charge subsidiary battery SB.
Though not shown, subsidiary electric components receiving power supply from subsidiary battery SB and driven thereby include an electrical control unit (ECU) controlling running of the vehicle, lighting, air conditioner, power window and audio system.
Among the vehicles having the motor driving apparatus shown in FIG. 6 mounted thereon, particularly in a hybrid vehicle, the power stored in main battery MB is used for starting engine operation. Specifically, the electric power is supplied from the main battery MB to motor generator MG coupled to the engine (not shown), and by driving the motor generator MG as a motor, the engine operation is started.
Further, for the motor driving apparatus mounted on a hybrid vehicle, a configuration in which a starter motor is driven by using a subsidiary battery at the time of starting engine operation has been disclosed (for example, in Japanese Patent Laying-Open Nos. 11-332012, 10-75502 and 8-93517).
FIG. 7 is a schematic block diagram showing another example of the conventional motor driving apparatus described in Japanese Patent Laying-Open No. 11-332012.
Referring to FIG. 7, an engine 210 is connected to a front wheel 216 through a transmission 212 and an axle 214. By an output of engine 210, front wheel 216is driven.
Engine 210 is driven by a starter motor 230, and starter motor 230 is driven by electric power of a subsidiary battery 220. Subsidiary battery 220 is charged by electric power generated by an alternator 219 driven by the output of engine 210.
The electric power of subsidiary battery 220 is boosted by a DC/DC converter 232, and the boosted electric power is stored in a capacitor (or condenser) 224. From capacitor 224, the electric power is supplied to left and right wheel motors 226 through an inverter 234. Thus, rear wheels 228 are driven.
In the configuration described above, when an ignition switch (not shown) is turned on and system ECU 236 is activated, an engine start control is performed. Specifically, electric power is supplied from subsidiary battery 220 to starter motor 230, starter motor 230 rotates, and the rotating force causes cranking of engine 210. Further, when start of operation of engine 210 is confirmed, the system related to wheel motor 226 is activated and running control is performed.
With such a control for starting engine operation, however, if the amount of electricity stored in subsidiary battery 220 should decrease in cold climate or because of degraded battery performance, sufficient electric power would not be supplied to starter motor 230, resulting in lower performance of engine start.
Therefore, in the motor driving apparatus of FIG. 7, a connection switching apparatus 238 is provided to selectively connect starter motor 230 either to subsidiary battery 220 or to capacitor 224. This enables switching of power supply source applying electric power to starter motor 230 between subsidiary battery 220 and capacitor 224, ensuring reliable starting of engine operation. Connection switching apparatus 238 is controlled by system ECU 236.
In a hybrid vehicle mounting the conventional motor driving apparatus as described above, a problem may arise that the vehicle system cannot be activated when the amount of electricity storage in the subsidiary battery decreases significantly, that is, when the subsidiary battery goes dead.
Specifically, in the motor driving apparatus shown in FIG. 6, the engine is started by driving motor generator MG as a motor. However, since control unit 120 controlling the motor driving apparatus as a whole uses the subsidiary battery SB as the power source, when the subsidiary battery goes dead, system relays SR1 and SR2 are not turned on, and therefore, electric power supply from main battery MB to boosting converter 100 and DC/DC converter 110 would be stopped. Therefore, motor generator MG cannot be driven and the engine cannot be started.
In the motor driving apparatus shown in FIG. 7, when the amount of electricity stored in subsidiary battery 220 decreases, though it is possible to supply electric power from capacitor 224 to starter motor 230 by using connection switching apparatus 238, system ECU 236 controlling the connection switching apparatus 238 would be inoperative when the subsidiary battery goes dead, making it difficult to start engine operation.
As described above, in either of the conventional motor driving apparatuses shown in FIGS. 6 and 7, run-out of subsidiary battery hinders starting of engine operation. Therefore, the driver of the vehicle must charge the subsidiary battery as soon as possible using a charging facility, as a countermeasure to the run-out of subsidiary battery.
On the other hand, even though the high-voltage main battery MB used for running the vehicle holds a sufficient amount of electricity to drive the motor generator MG, there is no means for effectively use the stored electricity when the vehicle system cannot be activated.
Japanese Patent Laying-Open No. 8-93517 discloses means for inhibiting, when the engine cannot be started as the voltage of subsidiary battery goes low, re-starting of operation of the starter motor of relatively large power consumption and allowing running with the electric power stored in the battery for running. Only with the battery for running, however, the range of running is significantly limited, and therefore, it is not guaranteed whether travel to a maintenance shop or the like, where charging facility is available, is possible or not.
Therefore, the present invention was made to solve such problems and its object is to provide a motor driving apparatus capable of driving the motor in a simple and reliable manner, even when the subsidiary battery goes dead.